This invention concerns improvements in or relating to infra-red optical systems.
There are a number of situations where an optical beam is required to pass through an aperture or to be reflected by a mirror surface and then divided to enter two separate detector channels. There are also some situations where it is required to combine two entrance beams to travel to a common detector. In each case a beam dividing or combining device is required, commonly referred to as a beam-splitter.
A beam-splitter may have a surface coating which is partially reflective, e.g. is wavelength selective in its reflection/transmission characteristics so that a chosen part of the incident waveband is strongly reflected while the rest of the waveband is transmitted. Such surface coating requires a supporting substrate and this can give rise to problems.
Beam-splitters are fairly commonly used for the visual waveband and can take a number of forms. One of the simplest is a parallel faced plate which carries the partially reflective coating. The plate is tilted at an appropriate angle to the entrance beam and the geometry of the optical system arranged around the resulting reflected and transmitted beams. This may be generally satisfactory if the light beam incident on the plate is collimated but if it is not, e.g. if the light beam is already inside the optical system and converging towards an intermediate image, then the tilted plate will introduce asymmetric aberration. The magnitude is generally dependent on the numerical aperture of the beam and the thickness of the plate and in some circumstances these may be kept sufficiently small for the introduced error to be tolerable, but in others they may not. Thinning of the plate can lead to further problems. A pellicle, which can be considered as an example of a very thin plate, needs to be isolated from vibration to operate satisfactorily and so has limited applications.
In view of these problems with a simple plate-like device, the beam-splitter arrangement frequently adopted for the visible waveband is a parallel sided block which is introduced into the beam with its faces orthogonal to the optical axis and which has an internal face, inclined to the optical axis, which carries the partially reflective coating. The prism components of the block are cemented together so that the beam transmitted through the partially reflecting internal interface effectively encounters a simple parallel sided block. The internally reflected optical axis is arranged to pass normally through the relevant exit face so that the reflected beam also, in effect, encounters a simple parallel sided block. The block may be cubic in form and is sometimes referred to as a beam-splitting cube.
Difficulties arise with beam-splitting cubes for infra-red radiation primarily because of the properties of infra-red transmitting optical materials. Firstly, the large physical size of typical infra-red optical systems tends to lead to long optical path lengths, which usually implies serious losses by absorption, as well as the considerable volume and mass leading to high cost and excessive weight. Secondly, infra-red materials generally have relatively high values of refractive index, say between 2 and 4, so the index of the material at the coated interface must be sufficiently large to prevent total internal reflection occurring there. In visual systems the visible light transmitting materials have lower refractive index values and the prism components are cemented together to form the block or cube, but there are difficulties in finding suitable infra-red cements. It will thus be seen that beam-splitting in infra-red optical systems presents problems which cannot generally be solved by simply adopting the solutions of visible light systems.